The present invention is directed to a tool bit for insertion into a tool bit chuck in a manually operated tool used for cutting and/or percussion drilling. The tool bit has an axially extending chucking shank with two locking grooves located substantially diametrically opposite one another and having closed ends spaced apart in the axial direction. In addition, two rotary entrainment grooves open at a free end of the chucking shank are located substantially diametrically opposite one another and the rotary entrainment grooves have different cross-sectional areas and openings in the outside surface of the chucking shank of different circumferential dimensions.
As disclosed in DE-PS 25 51 125 large numbers of tool bits for manually operated tools are in wide use throughout the world and have chucking shanks with two locking grooves located diametrically opposite one another and two rotary entrainment grooves also located diametrically opposite one another. The manually operated tools have tool chucks for receiving such tool bits. The tool bit chucks have two axially extending entrainment strips diametrically opposite one another which cooperate with the rotary entrainment grooves open at the free end of the chucking shank for transmitting torque to the tool bit. The cross-sectional area of the rotary entrainment grooves located diametrically opposite one another are equal and, therefore, the cross-sections of the entrainment strips cooperating with them are also of the same size.
In addition to torque transmission means, a tool bit chuck of the manually operated tools also have means for axially securing the tool bits. Particularly high requirements are not needed for axially securing the tool bits, since it is only necessary to assure that the tool bit is retained in the tool bit chuck during operation, for instance, that it does not fall out of the chuck due to its weight or is not separated from the chuck when the manually operated tool along with the tool bit is removed from a structural component in which it is being used.
Accordingly, the tool bit chucks use radially displaceable locking elements for effecting axial securement and such locking elements seat against the axially closed ends of the locking grooves in the chucking shank. The locking elements can be shaped as balls or rollers. A single radially displaceable locking element can be used or two such locking elements can be located diametrically opposite one another.
At the present time the requirements for torque transmission are continuously increasing, particularly with the use of ever increasing working diameters of the tool bits along with the increase in the output of the manually operated tools being utilized. As a result, it has been noted that the previously known rotary entrainment grooves along with the cooperating entrainment strips are no longer sufficient for transmitting the required high torques. Early wear of the chucking shank before the normal wear of the remaining parts of the tool bit has taken place. To overcome these problems it has been proposed in DE-A-39 41 646 to employ rotary entrainment grooves of larger cross-section. As a result, this involves the enlargement and strengthening of the entrainment strip in the tool bit chuck cooperating with the entrainment groove.
The advantage gained by the above mentioned cross-sectional enlargement is enhanced in DE-A-39 41 646 by the use of a third rotary entrainment groove in addition to the rotary entrainment grooves located diametrically opposite one another where one of such grooves has a larger cross-section.
By using this last mentioned arrangement, considerable advantages are gained in transmitting larger torques. A considerable disadvantage exists, however, in that the third rotary entrainment groove takes the place of one locking groove. Accordingly, the tool bits provided with the third rotary entrainment groove can not be used in tools where the tool bit chuck has two locking elements located diametrically opposite one another. Manually operated tools having such tool bit chucks are, however, in very wide spread use.
If the third rotary entrainment groove is dimensioned so that the displacement of a second locking element of a tool bit chuck is not obstructed, the possibility exists of faulty locking of such tool bits in a tool bit chuck with only one locking element and with entrainment strips having the same cross-section. Such faulty locking occurs when the tool bit is wrongly inserted by turning through 180.degree., so that it is not axially secured and may drop out of the tool chuck. Manually operated tools with such chucks are also widely used world wide.